Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, charged-coupled devices (CCDs), solar cells, and digital micro-mirror devices (DMDs).
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual projections for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
Semiconductor devices exploit the electrical properties of semiconductor materials. The atomic structure of semiconductor material allows its electrical conductivity to be manipulated by the application of an electric field or base current or through the process of doping. Doping introduces impurities into the semiconductor material to manipulate and control the conductivity of the semiconductor device.
A semiconductor device contains active and passive electrical structures. Active structures, including bipolar and field effect transistors, control the flow of electrical current. By varying levels of doping and application of an electric field or base current, the transistor either promotes or restricts the flow of electrical current. Passive structures, including resistors, capacitors, and inductors, create a relationship between voltage and current necessary to perform a variety of electrical functions. The passive and active structures are electrically connected to form circuits, which enable the semiconductor device to perform high-speed calculations and other useful functions.
Semiconductor devices are generally manufactured using two complex manufacturing processes, i.e., front-end manufacturing, and back-end manufacturing, each involving potentially hundreds of steps. Front-end manufacturing involves the formation of a plurality of die on the surface of a semiconductor wafer. Each semiconductor die is typically identical and contains circuits formed by electrically connecting active and passive components. Back-end manufacturing involves singulating individual semiconductor die from the finished wafer and packaging the die to provide structural support and environmental isolation. The term “semiconductor die” as used herein refers to both the singular and plural form of the words, and accordingly can refer to both a single semiconductor device and multiple semiconductor devices.
One goal of semiconductor manufacturing is to produce smaller semiconductor devices. Smaller devices typically consume less power, have higher performance, and can be produced more efficiently. In addition, smaller semiconductor devices have a smaller footprint, which is desirable for smaller end products. A smaller semiconductor die size can be achieved by improvements in the front-end process resulting in semiconductor die with smaller, higher density active and passive components. Back-end processes may result in semiconductor device packages with a smaller footprint by improvements in electrical interconnection and packaging materials.
Another goal of semiconductor manufacturing is to reduce a cost of making semiconductor devices. One back-end manufacturing technique employed to reduce the cost of semiconductor devices is the use of a molded underfill (MUF) process instead of a capillary underfill process (CUF). CUF is traditionally used as a first step in a two step process to fill a gap between a semiconductor die and a package substrate using an underfill material. Second, an encapsulant material is used to over mold or encapsulate the semiconductor die and package substrate. On the other hand, MUF is a simpler and more cost effective process that uses a single step approach to both underfill and over mold a semiconductor die in a single encapsulating process. After a semiconductor device has been encapsulated with MUF, the semiconductor device is inspected to detect flaws or defects within the device in order to eliminate potentially unreliable parts and to maintain quality assurance. One defect that occurs in semiconductor devices having undergone a MUF process is the formation of voids in the MUF or encapsulant around the semiconductor die. Another defect is the formation of cracks between bump structures and a dielectric layer, including low dielectric constants (low-k) cracks, which are commonly referred to as “white bumps” due to the appearance of the bumps in an acoustic or sound based scan such as scans using C-SAM. The need to inspect semiconductor devices for voids, cracks, and delamination is also a concern for semiconductor devices including a layer of polyimide (PI) or a PI coated substrate or wafer.
In addressing the goals of both producing smaller semiconductor devices and reducing packaging costs through the use of MUF, detection of defects using C-SAM is problematic. For low profile packages, e.g., packages made with a mold thickness of about 250 μm and with a semiconductor die thickness of about 70 μm, traditional C-SAM methods are unable to detect the presence of voids, white bumps, and low-k cracks. Packages with MUF and formed using a PI coated semiconductor wafer also limit the detection of voids, white bumps, and low-k cracks by traditional C-SAM methods. An inability to identify defects in MUF packages means a reduced ability to eliminate potentially unreliable parts, maintain quality assurance, and reduce semiconductor device failures.